board: engicam: Fix the ethernet clock initialization

According to the SOM and reference board the clock
can be taken from the external pin or provided from
ENET_REF_CLK. Add a new function that make the proper
set according the board type.

Signed-off-by: Michael Trimarchi <michael@amarulasolutions.com>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>